Reusing old floppy *drives*

Chris Elmquist chrise at
Tue Mar 23 12:25:10 CDT 2010

On Tuesday (03/23/2010 at 12:44PM +0000), Liam Proven wrote:
> I have read, somewhere, that it is possible to connect HD floppy
> drives to elderly kit that is only expecting DD drives, and use them
> for just DD operation.
> More specifically, I'd like to resurrect my ancient Sinclair Spectrum
> with its MGT DISCiPLE disk interface. Way back in the 1980s, I used
> 5¼" media, as they were still cheaper. However, now, I daresay I won't
> find any new 5¼" media and even 3½" DD media are getting tricky.
> So what I was considering doing was connecting a pair of old PC floppy
> drives - 3½" HD (1.4MB) floppies - and quite possibly using 1.4MB
> media, but only formatting them to 720K and using them as 720K. IIRC,
> the drives permit this.
> The track widths are the same - both are 80t drives - it's just that
> DD used 9 sectors per track and HD 18.
> Is this likely to work? I realise I'll need to find drives with a unit
> selection jumper on them, so I can set one as 0 and the other as 1; no
> twists in the cables this far back, & I'm guessing that a twisted
> cable wouldn't work with a controller that wasn't expecting it...?

I did a thing for the Heathkit H8/H89 world that emulates 10-hole,
hard-sector floppies using 720K, 3.5" drives.  An AVR micro synthesizes
the missing sector holes and then the controller thinks it is talking
to an age-old 5.25" drive with 10-sector media installed.

To make it work though, I had to use 3.5" drives that also support 720K
format.  That means that they have to have the sensor that detects the
density hole in the upper left corner of the 3.5" media.  When that hole
is blocked, the media is supposed to be 720K and this causes the drive
to run at 250Kbps instead of the normal 500Kbps rate it does when doing
1.44MB mode.

We have used 1.44MB media with this scheme by putting a piece of black
tape over the hole in the upper left corner of the 1.44MB floppy thereby
fooling the drive into thinking 720K media is installed.  It works OK.
Nobody has done any longevity studies or anything like that but the gut
feel is that these 3.5", whether they are true 720K or hacked 1.44MB,
are more robust and will last longer than the 10-hole hard-sector 5.25"
we are replacing and can't really buy any longer.

With regard to the drive selects, I chose to leave the drives alone--
leave them strapped like they would be if they went into a PC and I
even use the PC style cable with crossed selects and then I adapt the
selects from the legacy host controller to line up with the selects in
this "PC world".


Chris Elmquist

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